>I have been doing contract work for over 10 years on an hourly basis. The client's generally pay quickly and it's worked out well for all.
>
>Now I have a new client who wanted a fixed price bid. This is the first of at least 2 projects I'll be doing for them, probably more in the future. This first contract is rather small, and the next one is bigger.
>
>The question is; should I ask for a retainer? If so, how much? If not, how do I ensure that the client won't renege and leave me with nothing to show for it?

That seems like a sign for trouble. Ask 1/3 up front before a single line of code is written. If he refuses that arrangement, or asks for it to be lowered, watch out, he never wanted to pay you the full amount in the first place. I learnt my lesson.